1. Field of the Invention
This invention relates to an apparatus for sensing the concentration of hydrocarbons (HC) and carbon monoxide (CO) in a gas which may not contain oxygen, e.g., the exhaust gas from an internal combustion engine, using an apparatus having a sensor which accurately responds to HC and CO only when oxygen is present in the gas.
2. Description of the Related Art
Automotive solid state sensors for on-vehicle measurement of HC and CO in the exhaust gas may be useful for a number of applications such as optimization of engine operation with respect to emission of pollutants (HC, CO and NO.sub.x), fuel economy and drivability, detection of cylinder misfires, and monitoring the performance of catalysts used therein. Relatively simple and inexpensive solid state sensors for detection of combustibles including HC and CO are commercially available. These include resistive-type sensors and calorimetric-type sensors.
The resistive-type sensors measure the change in the electrical resistance of an appropriate material as a result of the interaction of the surface of the material with the combustibles. Several different materials including ceramics and polymers have been used for resistive-type sensors. For automotive exhaust applications, however, sensors based on metal oxides are preferable because these materials are more stable and durable in the automotive environment which includes high temperatures, oxidizing and reducing conditions, vibrations and presence of many contaminants. The most popular sensors of this kind are those based on SnO.sub.2. In fact, commercial SnO.sub.2 devices are made by Figaro Inc. and millions of these sensors are sold worldwide every year. These sensors are generally nonselective, that is, they respond to more than one combustible. However, by appropriate control of additives and sensor microstructure, some degree of selectivity to certain gases may be achieved.
The calorimetric-type sensors measure the rise in the temperature of an appropriate material as the result of the exothermic oxidation of the combustibles on the surface of this material or another material in contact with the first material. Examples of such materials are noble metals such as Pt or Pd. In general, these sensors are also nonselective, although, in some cases, some selectivity may be achieved by filtering or by differential measurements.
The HC and CO sensors of the prior art, however, require the presence of oxygen in the measurement gas for proper device operation. Resistive-type sensors such as SnO.sub.2 sensors generally require a large amount of oxygen for stable and reproducible operation. For the calorimetric-type sensors where during operation the HC and CO are oxidized prior to measurement, it is found desirable to provide oxygen in excess of that required for the complete oxidation. This excess of oxygen desirably increases the oxidation efficiency and hence operation of the sensor.
The requirement that oxygen be present in the measurement gas, and generally in excess amounts, for proper operation of these sensors substantially limits the usefulness of these sensors. When an automobile engine is operated with lean air-to-fuel mixtures, the exhaust gas always contains excess oxygen, its concentration increasing with increasing air-to-fuel ratio. On the other hand, when the engine is operated with fuel rich air-to-fuel mixtures, the amount of oxygen in the exhaust gas is very small or essentially nonexistent. Consequently, the sensors of the prior art are of limited usefulness when the air-to-fuel ratio of the engine is varied over a wide range including rich values, unless, for example, oxygen is injected into the exhaust gas as from ambient air. However, it has been found that these sensors operate optimally when a controlled amount of oxygen is provided to the sensor and controlling the amount of ambient air added to the exhaust gas is difficult if not impossible. These are some of the problems that the present invention overcomes.
Advantageously, this invention comprises an apparatus which contains one of these combustibles sensors yet can measure HC and CO in the exhaust gas even when it does not contain oxygen. This apparatus is thus able to operate accurately to measure HC and CO in exhaust gas under all engine operating conditions, from very rich air-to-fuel mixtures (absence of oxygen) to very lean air-to-fuel mixtures (abundance of oxygen) without adding ambient air to the exhaust gas but by pumping an amount of oxygen into the apparatus. According to the present invention, this oxygen can be added to the sensor in precisely controlled amounts. The present invention apparatus thus overcomes deficiencies of prior art sensors.